Radiosondes are small radio transmitters which are tethered to hydrogen or helium filled balloons and launched into the upper atmosphere to collect atmospheric data and to transmit this data to a remote data collection location. Radiosondes are used to measure temperature, air pressure, wind speed, ozone levels and humidity. The National Weather Service, in conjunction with weather services all over the world, simultaneously launches radiosondes in order to form a measurement grid of the upper atmosphere. These launches typically occur twice daily at twelve-hour intervals.
Launching each radiosonde typically requires two people and can take up to two hours to complete. First, the balloon is manually inflated. The balloons can be either three-foot diameter, 300-gram balloons or six-foot diameter 600-gram balloons, depending on the weather conditions and the size of the radiosonde being launched. The balloons are formed of very thin latex and must be inflated very slowly in order to minimize the risk of rupturing. The inflation takes approximately ten to fifteen minutes at which time the buoyancy of the balloon is manually measured by the technician. Generally, a three-foot balloon must have approximately 300-400 grams of lift capacity and a six-foot balloon must have approximately 1 kg of lift capacity. Once the balloon is inflated, it is sealed and the radiosonde is tied to the balloon with approximately 60-100 feet of line.
Preparing the radiosonde for launching involves unpacking the sonde and correctly positioning the hygrometer, pressure sensor, transmitter and antenna from the sonde to ensure proper operation of each measurement element. The sonde includes a battery which is sealed and dry during shipping and must be soaked in water in order to charge the battery. Once soaked, the battery requires several minutes to charge. A technician then checks the battery voltage using a light bulb, turns the transmitter on and registers the ID code of the sonde by scanning the bar code which identifies each sonde during its flight. The technician then checks the function of each of the measuring elements, e.g., the thermometer, hygrometer, pressure sensor, to ensure that each is functioning properly.
This process is time consuming, requires a team of technicians and can be extremely hazardous and difficult to perform in remote launch sites such as on mountain ranges and in deserts.